The invention relates to improvements in powder processing of intermetallic materials such as iron aluminides.
Iron base alloys containing aluminum can have ordered and disordered body centered crystal structures. For instance, iron aluminide alloys having intermetallic alloy compositions contain iron and aluminum in various atomic proportions such as Fe3Al, FeAl, FeAl2, FeAl3, and Fe2Al5. Fe3Al intermetallic iron aluminides having a body centered cubic ordered crystal structure are disclosed in U.S. Pat. Nos. 5,320,802; 5,158,744; 5,024,109; and 4,961,903. Such ordered crystal structures generally contain 25 to 40 atomic % Al and alloying additions such as Zr, B, Mo, C, Cr, V, Nb, Si and Y.
A 1990 publication in Advances in Powder Metallurgy, Vol. 2, by J. R. Knibloe et al., entitled xe2x80x9cMicrostructure And Mechanical Properties of P/M Fe3Al Alloysxe2x80x9d, pp. 219-231, discloses a powder metallurgical process for preparing Fe3Al containing 2 and 5% Cr by using an inert gas atomized powder. This publication explains that Fe3Al alloys have a DO3 structure at low temperatures and transform to a B2 structure above about 550xc2x0 C. To make sheet, the powders were canned in mild steel, evacuated and hot extruded at 1000xc2x0 C. to an area reduction ratio of 9:1. After removing from the steel can, the alloy extrusion was hot forged at 1000xc2x0 C. to 0.340 inch thick, rolled at 800xc2x0 C. to sheet approximately 0.10 inch thick and finish rolled at 650xc2x0 C. to 0.030 inch. According to this publication, the atomized powders were generally spherical and provided dense extrusions and room temperature ductility approaching 20% was achieved by maximizing the amount of B2 structure.
A 1991 publication in Mat. Res. Soc. Symp. Proc., Vol. 213, by V. K. Sikka entitled xe2x80x9cPowder Processing of Fe3Al-Based Iron-Aluminide Alloys,xe2x80x9d pp. 901-906, discloses a process of preparing 2 and 5% Cr containing Fe3Al-based iron-aluminide powders fabricated into sheet. This publication states that the powders were prepared by nitrogen-gas atomization and argon-gas atomization. The nitrogen-gas atomized powders had low levels of oxygen (130 ppm) and nitrogen (30 ppm). To make sheet, the powders were canned in mild steel and hot extruded at 1000xc2x0 C. to an area reduction ratio of 9:1. The extruded nitrogen-gas atomized powder had a grain size of 30 xcexcm. The steel can was removed and the bars were forged 50% at 1000xc2x0 C., rolled 50% at 850xc2x0 C. and finish rolled 50% at 650xc2x0 C. to 0.76 mm sheet.
A paper by V. K. Sikka et al., entitled xe2x80x9cPowder Production, Processing, and Properties of Fe3Alxe2x80x9d, pp. 1-11, presented at the 1990 Powder Metallurgy Conference Exhibition in Pittsburgh, Pa., discloses a process of preparing Fe3Al powder by melting constituent metals under a protective atmosphere, passing the metal through a metering nozzle and disintegrating the melt by impingement of the melt stream with nitrogen atomizing gas. The powder had low oxygen (130 ppm) and nitrogen (30 ppm) and was spherical. An extruded bar was produced by filling a 76 mm mild steel can with the powder, evacuating the can, heating 1xc2xd hour at 1000xc2x0 C. and extruding the can through a 25 mm die for a 9:1 reduction. The grain size of the extruded bar was 20 xcexcm. A sheet 0.76 mm thick was produced by removing the can, forging 50% at 1000xc2x0 C., rolling 50% at 850xc2x0 C. and finish rolling 50% at 650xc2x0 C.
A publication by A. LeFort et al., entitled xe2x80x9cMechanical Behavior of FeAl40 Intermetallic Alloysxe2x80x9d presented at the Proceedings of International Symposium on Intermetallic Compoundsxe2x80x94Structure and Mechanical Properties (JIMIS-6), pp. 579-583, held in Sendai, Japan on Jun. 17-20, 1991, discloses various properties of FeAl alloys (25 wt. % Al) with additions of boron, zirconium, chromium and cerium. The alloys were prepared by vacuum casting and extruding at 1100xc2x0 C. or formed by compression at 1000xc2x0 C. and 1100xc2x0 C. This article explains that the excellent resistance of FeAl compounds in oxidizing and sulfidizing conditions is due to the high Al content and the stability of the B2 ordered structure.
A publication by D. Pocci et al., entitled xe2x80x9cProduction and Properties of CSM FeAl Intermetallic Alloysxe2x80x9d presented at the Minerals, Metals and Materials Society Conference (1994 TMS Conference) on xe2x80x9cProcessing, Properties and Applications of Iron Aluminidesxe2x80x9d, pp. 19-30, held in San Francisco, Calif. on Feb. 27-Mar. 3, 1994, discloses various properties of Fe40Al intermetallic compounds processed by different techniques such as casting and extrusion, gas atomization of powder and extrusion and mechanical alloying of powder and extrusion and that mechanical alloying has been employed to reinforce the material with a fine oxide dispersion. The article states that FeAl alloys were prepared having a B2 ordered crystal structure, an Al content ranging from 23 to 25 wt. % (about 40 at %) and alloying additions of Zr, Cr, Ce, C, B and Y2O3. The article states that the materials are candidates as structural materials in corrosive environments at high temperatures and will find use in thermal engines, compressor stages of jet engines, coal gasification plants and the petrochemical industry.
A publication by J. H. Schneibel entitled xe2x80x9cSelected Properties of Iron Aluminidesxe2x80x9d, pp. 329-341, presented at the 1994 TMS Conference discloses properties of iron aluminides. This article reports properties such as melting temperatures, electrical resistivity, thermal conductivity, thermal expansion and mechanical properties of various FeAl compositions.
A publication by J. Baker entitled xe2x80x9cFlow and Fracture of FeAlxe2x80x9d, pp. 101-115, presented at the 1994 TMS Conference discloses an overview of the flow and fracture of the B2 compound FeAl. This article states that prior heat treatments strongly affect the mechanical properties of FeAl and that higher cooling rates after elevated temperature annealing provide higher room temperature yield strength and hardness but lower ductility due to excess vacancies. With respect to such vacancies, the articles indicates that the presence of solute atoms tends to mitigate the retained vacancy effect and long term annealing can be used to remove excess vacancies.
A publication by D. J. Alexander entitled xe2x80x9cImpact Behavior of FeAl Alloy FA-350xe2x80x9d, pp. 193-202, presented at the 1994 TMS Conference discloses impact and tensile properties of iron aluminide alloy FA-350. The FA-350 alloy includes, in atomic %, 35.8% Al, 0.2% Mo, 0.05% Zr and 0.13% C.
A publication by C. H. Kong entitled xe2x80x9cThe Effect of Ternary Additions on the Vacancy Hardening and Defect Structure of FeAlxe2x80x9d, pp. 231-239, presented at the 1994 TMS Conference discloses the effect of ternary alloying additions on FeAl alloys. This article states that the B2 structured compound FeAl exhibits low room temperature ductility and unacceptably low high temperature strength above 500xc2x0 C. The article states that room temperature brittleness is caused by retention of a high concentration of vacancies following high temperature heat treatments. The article discusses the effects of various ternary alloying additions such as Cu, Ni, Co, Mn, Cr, V and Ti as well as high temperature annealing and subsequent low temperature vacancy-relieving heat treatment.
A publication by D. J. Gaydosh et al., entitled xe2x80x9cMicrostructure and Tensile Properties of Fe40 At.Pct. Al Alloys with C, Zr, Hf and B Additionsxe2x80x9d in the September 1989 Met. Trans A, Vol. 20A, pp. 1701-1714, discloses hot extrusion of gas-atomized powder wherein the powder either includes C, Zr and Hf as prealloyed additions or B is added to a previously prepared iron-aluminum powder.
A publication by C. G. McKamey et al., entitled xe2x80x9cA review of recent developments in Fe3Al-based Alloysxe2x80x9d in the August 1991 J. of Mater. Res., Vol. 6, No. 8, pp. 1779-1805, discloses techniques for obtaining iron-aluminide powders by inert gas atomization and preparing ternary alloy powders based on Fe3Al by mixing alloy powders to produce the desired alloy composition and consolidating by hot extrusion, i.e., preparation of Fe3Al-based powders by nitrogen- or argon-gas atomization and consolidation to full density by extruding at 1000xc2x0 C. to an area reduction of xe2x89xa69:1.
U.S. Pat. Nos. 4,917,858; 5,269,830; and 5,455,001 disclose powder metallurgical techniques for preparation of intermetallic compositions by (1) rolling blended powder into green foil, sintering and pressing the foil to full density, (2) reactive sintering of Fe and Al powders to form iron aluminide or by preparing Nixe2x80x94Bxe2x80x94Al and Nixe2x80x94Bxe2x80x94Ni composite powders by electroless plating, canning the powder in a tube, heat treating the canned powder, cold rolling the tube-canned powder and heat treating the cold rolled powder to obtain an intermetallic compound.
U.S. Pat. No. 5,484,568 discloses a powder metallurgical technique for preparing heating elements by micropyretic synthesis wherein a combustion wave converts reactants to a desired product. In this process, a filler material, a reactive system and a plasticizer are formed into a slurry and shaped by plastic extrusion, slip casting or coating followed by combusting the shape by ignition.
U.S. Pat. Nos. 5,098,469 and 5,269,830 disclose techniques for preparing intermetallic alloy compositions by powder metallurgical techniques which include pressureless sintering. The ""469 patent discloses a four step pressureless sintering process for producing Nixe2x80x94Alxe2x80x94Ti intermetallic aluminide alloys wherein a compact of nickel powder and prealloyed aluminide powder is heated without cool down steps and with a heating rate of 10xc2x0 C. per minute between the processing steps. The ""830 patent discloses a pressureless sintering process for producing Fe3Al and FeAl compounds wherein elemental powders of iron and aluminum are heated under conditions of temperature and pressure to produce an exothermic reaction and densification is achieved by sintering in vacuum or by pressure assisted densification by heating during compression. According to the ""830 patent, pressureless sintering achieves near 75% of full density.
Based on the foregoing, there is a need in the art for an economical technique for preparing intermetallic compositions such as iron aluminides. For instance, conventional powder metallurgical techniques of preparing iron-aluminides include melting iron and aluminum and inert gas atomizing the melt to form an iron-aluminide powder, canning the powder and working the canned material at elevated temperatures or reaction synthesis can be used to react elemental powders of iron and aluminum. It would be desirable if iron-aluminide could be prepared by a powder metallurgical technique wherein it is not necessary to can the powder and wherein it is not necessary to subject the iron and aluminum to any hot working steps in order to form an iron-aluminide sheet product.
Other publications which disclose aluminide processing techniques include commonly-owned U.S. Pat. Nos. 5,595,706; 5,620,651; 5,976,458; 6,030,472; and 6,033,623.
The invention provides a method of manufacturing an iron aluminide intermetallic alloy composition by a powder metallurgical technique, comprising steps of forming a powder mixture comprising aluminum powder and iron powder, heating the powder mixture so as to react the aluminum powder and the iron powder to form a first reacted compact containing Fe2Al5, free-aluminum and free-iron, heating the first reacted compact so as to react the free-iron with the free-aluminum and/or the Fe2Al5 to form a second reacted compact containing FeAl, Fe2Al5 and free-iron; and heating the second compact so as to react the free-iron with the FeAl and/or the Fe2Al5 to form a sintered compact containing FeAl.
The heating steps are preferably carried out in a vacuum or inert gas (e.g., argon or helium with or without minor additions of hydrogen) environment such that (1) the Fe2Al5 is formed by a solid state reaction without melting the aluminum powder and/or expansion of the first reacted compact due to volume change during formation of the Fe2Al5 is less than 10%, (2) the aluminum powder is completely melted during formation of the FeAl and/or expansion of the second reacted compact due to volume change during formation of the FeAl is less than 10%, (3) the FeAl initially forms as a layer between the iron powder and the Fe2Al5, and/or (4) expansion of the sintered compact due to volume change during formation of the FeAl is less than 10%. In a preferred process, the powder mixture is heated at a heating rate of less than 1xc2x0 C./min and/or the sintered compact is heated sufficiently to increase the density of the sintered compact to at least 90%, more preferably at least about 95% of the theoretical density. The process can include a step of pressing the powder mixture into a shaped article.
According to the process, the following reactions can sequentially occur during the heating steps: (1) Fe2Al5 is formed as a layer around the individual particles of the iron powder without melting of the aluminum powder, (2) the aluminum powder melts and diffuses into the iron powder, (3) some of the FeAl is formed by an interfacial reaction between the iron powder and the Fe2Al5, and (4) the balance of the FeAl is formed by a solid state diffusion.